3 ANSWERS

A teenager will act/react differently depending on household nudity up until that point, if they have other siblings and if they have been taught to be open with questions and such.
The healthy boundaries depend on the upbringing based on things like mentioned above.

There is nothing wrong with a teenage son seeing his mother naked. In some societies families bathe together. If a mother and son have a stable enough relationship, there should be no problem with the son seeing mom naked.